This invention relates to the process for making a trench box for protecting against a collapse of the sidewalls of an excavation. More specifically, the instant invention pertains to panels of the trench box.
It is conventional to fabricate a panel for a trench box using laterally spaced steel U-shaped longitudinal channels, or as an alternative using laterally spaced tubes, located between and attached to large flat steel plates. To reduce weight and material costs, the longitudinal channels or tubes are often spaced laterally a short distance. The longitudinal spaces between adjacent channels, or the gaps between adjacent tubes, are then closed locally by short spacers to provide patterned continuity and uniformity. Relatively, the spacers provide only nominal structural strength between adjacent longitudinal members. Lengthy spaces and gaps remain left where there are no spacers between adjacent channels or tubes. These spaces and gaps cause the panels to have discontinuous bending stiffness and strength required to resist external loads tending to bend the panel across its width.
To compensate for this deficiency, vertical stiffeners, which are usually extruded channels, are placed at spaced locations along the panel's length and secured to the longitudinal channels. Except at the locations of the vertical stiffeners, panels fabricated in this way lack uniform bending stiffness across the panel's width. Due to changes in the moment of inertia of the bending cross-section that occurs at each longitudinal space between adjacent longitudinal channels, bending loads are concentrated near the vertical stiffeners and short channels rather than being carried uniformly across the entire length of the panel. This concentration of loading lowers the structural efficiency of the panel and requires use of thicker metal and heavier cross sections to reach the stiffness and strength that would result if the full panel length were uniformly active in resisting lateral bending.
A large inventory of U-shaped longitudinal channels with various thicknesses and dimensions, or multiple tubes having different lengths, widths, wall thickness and diameters is required to properly engineer and assemble panels of this type so as to provide the needed structural strength for the particular situation. As a result, a manufacturer must either make a significant financial investment to keep an adequate inventory of longitudinal members to choose from or order the required members once appropriate sizes and dimensions are determined. In both cases, the manufacturer is either delayed or forced to incur increased overhead costs. This problem is aggravated with the escalation or unpredictable fluctuation in steel prices. Furthermore, because a conventional panel fabricated as described may have relatively large areas of overlapping thicknesses of the extruded, bent, shaped and/or formed members, the panel typically has unnecessary excess weight.
Still further, the complexity to fabricate and assemble such panels adds to the time, engineering and cost of assembly. One of the more significant problems faced when fabricating U-shaped longitudinal channels, for example, is the bending of a flat sheet on a brake press to make the required U-shape with appropriate leg or web dimension and spacing. More specifically, to increase the strength of the longitudinal member to resist bending along its length, engineering may require the U-shaped channel have a short width with long parallel legs, thereby increasing the webbing and overall thickness of the finished panel. With such a design, however, it becomes difficult, if not impractical, to bend sheet steel on a brake press to make the appropriately dimensioned U-shaped cross-section. With the closely aligned extended legs, the length of the first bent leg interferes with the brake press during the bending of the second leg, thereby preventing the needed 90° bend.
The alternative may be to use a tube with a larger diameter and thicker wall. However, this design makes for other engineering problems.
In order to eliminate the costs associated with specially fabricating and maintaining large inventories of U-shaped longitudinal channels and tubes for each of the multiple dimensioned profiles, it is desirable to provide a channel profile and assembly that can be easily and quickly fabricated to multiple dimensions from stock sheet steel, while providing more engineering versatility, improved structural strength and reliability. It is therefore an objective of the present invention to provide such a profile and fabricating mechanism.